Effects of storage-ring time structure on high-speed X-ray photon correlation spectroscopy

The recent availability of X-ray pixel-array detectors with nanosecond time resolution now allows X-ray photon correlation spectroscopy (XPCS) studies of nanosecond dynamics in materials. However, a storage-ring source of synchrotron radiation has temporal structure in the nanosecond domain which can contribute strongly to observed time correlation functions, especially for non-uniform fill patterns. When detectors can resolve them, such source effects extend out to tens of microseconds in typical XPCS analysis. Here it is shown how to characterize and remove the contribution from the source. Because the sample dynamics are uncorrelated with the source, the source time correlation function contributes simply as a multiplicative factor in the observed correlation function. Our results demonstrate that it is possible to measure the effect of source time structure from XPCS datasets and to remove it accurately, to reveal the dynamics of the sample into the nanosecond regime. Since non-uniform fill patterns are often used at storage-ring X-ray sources, this greatly extends the availability of beam time suitable for high-speed XPCS.